C++11 adds cbegin()/cend() functions for the same reason Qt has
constBegin()/constEnd(). This patch adds these functions to the
Qt containers with the same implementation as constBegin()/constEnd().
It also fixes the return types in the documentation of existing
constFind() functions (documentation only).
C++11 only adds cbegin()/cend() (and crbegin()/crend(), which Qt doesn't have).
In particular, it doesn't add cfind(), so I didn't supply these, even though
Qt comes with constFind().
This is a forward-port of https://qt.gitorious.org/qt/qt/merge_requests/1365.
Change-Id: Ida086b64246b24e25254eafbcb06c8e33388502b
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
d823646db3